1. Field of the Invention
The present invention relates to an AGC (automatic gain control) system, an AGC method, and a receiver using the AGC system. In particular, the present invention relates to an AGC system in a receiver of a CDMA (code division multiple access) system and a direct conversion system.
2. Description of the Related Art
In recent years, with the spread of a mobile phone and other wireless portable terminals, demands for downsized wireless devices, lower power consumption, and lower prices have been increasingly made. As one of wireless receiving systems that meets the above demands, there is a direct conversion system. The direct conversion system converts a received radio frequency (RF) signal into a baseband signal. In the case where the direct conversion system is used in a receiver of a wireless device that operates with a low power as with a mobile phone, it is important to remove a direct current (DC) offset voltage. The removal of the direct current offset component specific to the direct conversion system is disclosed in “Baseband circuit in a direct conversion receiver” of JP 2002-111764 A (laid open in Apr. 12, 2002).
For removing the direct offset component, the baseband circuit disclosed in the above-mentioned JP 2002-111764 A uses a filter; however, it needs a time attributable to a time constant of the filter for stabilizing the filter. Accordingly, there occurs an instability period of a received signal, that is, a noise generation period. It is necessary to suppress an influence of the instability period (noise generation period) on the signal as much as possible.
In general, in the receiver, it is necessary to change over the gain of an amplifier by using the AGC system. The amplifier connects to the filter of the receiver, and amplifies the baseband signal. When the gain is changed over, the function of removing the direct offset component is operated, and the above instability period occurs. In particular, in the DS-CDMA (direct sequence-code division multiple access)-based communication system, data is subjected to band spreading by a broadband spreading code. Accordingly, because the information is always transmitted, there is no temporal gap in received RF signal. As a result, the above instability period of the received signal leads to the deterioration of the receiving performance.
In addition, when the instable receiving state occurs at the same timing within a slot cyclically or at a specific position by a time slot (hereinafter referred to simply as “slot”) of the received signal, the receiving performance is remarkably deteriorated.